This application claims priority of Japanese Patent Application No. P11-136432 filed on May 17, 1999, and PCT Application PCT/JP00/00202 filed on Jan. 18, 2000, which are hereby incorporated by reference, to the extent permitted by law.
This invention relates to a disc-shaped multi-layered recording medium, having plural layers, carrying information signals thereon, and a method for producing the same.
In a field of information recording, an optical information recording system is used. The optical information recording system has a number of advantages, such that, in the optical information recording system, information signals can be recorded and/or reproduced without a recording and/or reproducing head contacting a recording medium, and a recording density higher by not less than one digit than in the magnetic recording system can be achieved, while respective memory configurations, such as replay-only, write once or overwrite type memory configuration, can be coped with. So, a wide range of application ranging from the industrial usage to domestic usage has been proposed for the optical information recording system as a system enabling realization of an inexpensive large-capacity file.
Especially among them are commonly used a high-density information recording medium, an optical video disc, etc. called a digital video disc or a digital versatile disc which are optical discs coping with replay-only memory configuration.
In this sort of the optical disc, a reflective film, as a thin metal film of e.g., aluminum, is formed on an optical disc substrate, as a transparent substrate, carrying a crest-and-recess pattern, such as pits and grooves, representing information signals. On this reflective film is formed a protective film for protecting this reflective film from moisture and oxygen in atmosphere. For reproducing the information from the optical disc, the reproducing light, such as laser light, is illuminated from the side optical disc substrate on the crest-and-recess pattern to detect the information based on the difference in light reflectance between the incident light and the reflected light.
For manufacturing this optical disc, an optical disc substrate, carrying the crest and recess pattern, is formed by a technique, such as injection molding, and a reflective film, comprised of a thin metal film, is formed by any suitable technique, such as vapor deposition. On this reflective film is applied a ultra-violet light curable resin to form a protective film.
In this optical disc, it is desired to realize a high recording capacity. In order to meet this demand, such an optical disc is proposed in which a crest-and-recess pattern is formed on one surface of an optical disc substrate, a semi-transparent film is formed thereon, a crest-and-recess pattern is again formed on this semi-transparent film, at an interval of tens of micrometers (xcexcm), and a reflective film is formed thereon, with the optical disc having a sum total of two information substrate layers.
It is desirable that the multi-layered disc-shaped recording medium, having these signal layers, be of large capacity and high productivity and be readily producible.
It is therefore an object of the present invention to provide a multi-layered information recording medium realizing a large recording capacity and which can be produced readily in larger quantities.
For accomplishing the above object, the present invention provides a multi-layered disc-shaped information recording medium having a plurality of signal recording layers each carrying information signals. In preparing the recording medium, a first substrate obtained on punching a sheet obtained in turn by having a signal recording pattern transcribed on it and by forming a semi-transparent film on a signal surface is bonded to a second substrate obtained on injection molding a synthetic resin material. The second substrate is prepared by having transcribed to it a signal recording pattern formed on a second stamper provided on an injection molding device. A reflective film is formed on a signal surface having the transcribed second substrate. The first and second substrates are bonded together on their signal recording layers by a transparent adherent layer.
The present invention also provides a method for producing a multi-layered disc-shaped information recording medium having plural layers each carrying a signal recording pattern, including a first substrate forming step of preparing a first substrate by transcribing a signal recording pattern formed on a first stamper onto a sheet to produce a signal surface, forming a semitransparent film on the signal surface and by punching the resulting sheet, a second substrate forming step of preparing a second substrate by transcribing a signal recording pattern formed by injection molding on a second stamper, and by forming a reflective film on a resulting signal surface, and a bonding step of bonding the first and second substrates by a transparent adherent layer.
In the first substrate forming step, the signal recording pattern is transcribed to the sheet from the first stamper and a semi-transparent film is formed on a resulting signal surface having the signal recording pattern transcribed thereon. The resulting sheet is punched to produce a first substrate. In the second substrate forming step, the signal recording pattern formed on the second stamper by injection molding is transcribed to form a signal surface and a reflective film is formed thereon to produce a second substrate.
In the first substrate forming step, a continuous sheet is used to prepare a first substrate. In the bonding step, the first and second substrates are bonded together by a transparent adherent layer to render it possible to produce multi-layered disc-shaped information recording mediums in large quantities. In the second substrate forming step, the second substrates can be produced in large quantities by injection molding. By using these steps, the multi-layered large-capacity disc-shaped information recording mediums can be readily produced in large quantities.
In the first substrate forming step, the first stamper is fed with resin and the first stamper is pressured and/or UV light rays are illuminated on the sheet of synthetic resin for transcribing the signal recording pattern from the first stamper to the sheet of synthetic resin. Moreover, in the first substrate forming step, the first stamper is pressured against the sheet of synthetic resin for directly transcribing the signal recording pattern to the sheet from the first stamper.
In the bonding step, a photo-curable resin is used as a transparent adherent layer, and the first and second substrates are rotated to control the thickness of the transparent adherent layer to unify the first and second substrates through the transparent adherent layer.
If, in the bonding step, UV light rays are illuminated on the transparent adherent layer as a point light source for the UV light rays which is moved from the inner rims towards the outer rims of the first and second substrates, the transparent adherent layer may be photo-cured by simply moving the light source for the UV light rays.
On the first stamper used in the present method, the signal recording pattern is produced as an inverted pattern from a pattern obtained on sputtering a mother stamper with metal.
The first and second substrates are bonded together using a pressure-sensitive adhesive.
The transparent adherent layer used and the resin for signal transcription are both dry photo-polymers.
The sheet making up the first substrate used in the present invention is formed by bonding a protective sheet to a pressure-sensitive sheet after transcribing the signal recording pattern from the first stamper to the pressure-sensitive sheet, removing the protective sheet for use as a first substrate, on which the second substrate is bonded.
The surface of the sheet used in the present invention, on which falls the laser light for recording and/or reproduction, is coated with a protective agent. This protective agent increases surface hardness or lowers the surface resistance or frictional coefficients.
The present invention also provides a method for producing a multi-layered disc-shaped information recording medium having plural layers each carrying a signal recording pattern. The method includes a first substrate forming step of preparing a first substrate by forming a signal recording pattern provided on a first stamper on a sheet and by forming a semi-transparent film on a signal surface carrying the signal recording pattern, and a semi-transparent film forming step of forming one or more semi-transparent films, having a signal recording pattern transcribed thereon, on the semi-transparent film of the first substrate by the interposition of a transparent adherent layer at a separation preferably of tens of xcexcm.
In producing a multi-layered disc-shaped information recording medium, having plural signal layers, a signal recording pattern is transcribed from the first stamper to a sheet in the first substrate forming step, and one or more semi-transparent films, now carrying the signal recording pattern, are formed on the semi-transparent film of the first substrate with the interposition of a transparent adherent layer at a separation of tens of xcexcm.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention and the claims.